A SSD is a data storage device using integrated circuit assemblies as memory to store data persistently. SSD technology uses electronic, interfaces compatible with traditional block input/output (I/O) hard disk drives, thus permitting simple replacement in common applications.
SSDs have no moving (mechanical) components. This distinguishes them from traditional electromechanical magnetic disks such as hard disk drives (HDDs) or floppy disks, which contain spinning disks and movable read/write heads. Compared with electromechanical disks, SSDs are typically more resistant to physical shock, run silently, have lower or faster access time, and less latency. However, while the price of SSDs has continued to decline over time, SSDs are still roughly seven to eight times more expensive per unit of storage than HDDs.
Recently, most SSDs use NAND (Not And electronic logic gate)-based flash memory, which retains data without power. For applications requiring fast access, but not necessarily data persistence after power loss, SSDs may be constructed from random-access memory (RAM). Such devices may employ separate power sources, such as batteries, to maintain data after power loss.
Key components of a SSD are the controller and the memory to store the data. The primary memory component in a SSD was traditionally dynamic random-access memory DRAM volatile memory, now more commonly NAND flash non-volatile memory. Other components play a less significant role in the operation of the SSD and vary among manufacturers.
Every SSD includes a controller that incorporates the electronics that bridge the NAND memory components to the host computer. The controller is an embedded processor that executes firmware-level code and is one of the most important factors of SSD performance. Some of the functions performed by the controller include: error-correcting code (ECC), wear leveling, bad block mapping, read scrubbing and read disturb management, read and write caching, garbage collection, and encryption.
Most SSD manufacturers use non-volatile NAND flash memory in the construction of their SSDs because of the lower cost compared with DRAM and the ability to retain the data without a constant power supply, ensuring data persistence through sudden power outages. Flash memory SSDs are slower than DRAM solutions, and some early designs were even slower than HDDs after continued use. This problem was resolved by controllers that came out later. Flash memory-based solutions are typically packaged in standard disk drive form factors (1.8-, 2.5-, and 3.5-inch), or smaller unique and compact layouts because of the compact memory.
Another component in higher performing SSDs is a capacitor or some form of battery. These are necessary to maintain data integrity such that the data in the cache can be flushed to the drive when power is dropped; some may even hold power long enough to maintain data in the cache until power is resumed. In the case of MLC (multi level cell) flash memory, a problem called lower page corruption can occur when MLC flash memory loses power while programming an upper page. The result is that data written previously and presumed safe can be corrupted if the memory is not supported by a super capacitor in the event of a sudden power loss. This problem does not exist with SLC flash memory.
The size and shape of any device is largely driven by the size and shape of the components used to make that device. If an SSD is made up of various interconnected integrated circuits (ICs) and an interface connector, then its shape could be virtually anything imaginable because it is no longer limited to the shape of rotating media drives. Some solid state storage solutions come in a larger chassis that may even be a rack-mount form factor with numerous SSDs inside. They would all connect to a common bus inside the chassis and connect outside the box with a single connector.
For general computer use, the 2.5-inch form factor (typically found in laptops) is the most popular. For desktop computers with 3.5-inch hard disk slots, a simple adapter plate can be used to make such a disk fit. Other types of form factors are more common in enterprise applications. An SSD can also be completely integrated in the other circuitry of the device, as in certain Apple devices. Further, mSATA and M.2 form factors are also gaining popularity, primarily in laptops.